Fluororesins are excellent in such characteristics as thermal stability, chemical resistance, solvent resistance and insulating properties and therefore, when molded by melt extrusion, can give such products as tubes, electric wire coverings, pipes and filaments. In particular, fluororesins comprising tetrafluoroethylene [TFE]/hexafluoropropylene [HFP]-based copolymer [FEP] are low in dielectric constant and dielectric loss tangent and thus have good insulating properties and therefore are suitably used in the field of covering of such electric conductors as cables and wires.
In molding electric wire coverings, for instance, a material is required which makes it possible to increase the molding speed and, at the same time, reduce the proportion of defective moldings so that the productivity may be improved and the cost may be reduced.
For the purpose of reducing the phenomenon of melt fracture among molding defects and improving the critical extrusion speed, it has been proposed that a polytetrafluoroethylene [PTFE] be incorporated in a FEP to a PTFE content of 0.01 to 5% by weight (cf. e.g. Patent Document 1: Japanese Kokai (Laid-open) Publication S52-98761) and, for the purpose of diminishing covering breakage in the process of covering electric wires, it has been proposed that 0.03 to 2 parts by weight of PTFE be incorporated in 100 parts by weight of a FEP or a terpolymer thereof with a perfluoro(alkyl vinyl ether) (cf. e.g. Patent Document 2: International Publication WO 03/22922 and Patent Document 3: International Publication WO 03/22923).
In these technologies, it is indeed disclosed that PTFE be incorporated in the form of a dispersion. As for the FEP and terpolymer mentioned above, however, they are added each in the form of a powder; there is no description of their being incorporated in the form of dispersions.
These technologies are indeed effective to some extent in reducing the size of resin lumps formed between the covering resin and conductor but have a problem in that they are insufficient for reducing the frequency of lump formation.
It has been proposed that in preparing a melt-processable composition comprising a FEP and 0.01 to 5% by weight, relative to the FEP, of a high-molecular-weight perfluoropolymer having a melting point higher by at least 20° C. than the FEP, the FEP and perfluoropolymer be compounded each in the form of a dispersion (cf. e.g. Patent Document 4: Japanese Kohyo (Laid-open under PCT) Publication 2004-502853). As regards this perfluoropolymer, mention is made only of TFE/HFP copolymers containing 2 to 20% by weight of HFP units as essential units. No mention is made of PTFE.
Allegedly, this composition carries away lumps and prevents them from accumulating in wire covering molding. However, like the above-mentioned technology consisting in compounding a FEP powder with PTFE, there is a problem in that it does not reduce the frequency of lump formation. Lumps small in size cause increases in capacitance fluctuation and the problem of deteriorations in electric characteristics of final products is produced thereby.
For the purpose of attaining stable moldability by reducing the fluctuations in wire covering diameter in wire covering molding, a FEP copolymer has been proposed which shows a relatively high die swell ratio of 5 to 20% and shows a melt flow rate within a specified range (cf. e.g. Patent Document 5: International Publication WO 01/36504). However, there is no description at all of the addition of a high-molecular-weight perfluoropolymer such as PTFE.
As a fluororesin serving as a material for moldings excellent in surface smoothness, a TFE/fluoroalkyloxytrifluoroethylene copolymer composition containing 0.01 to 30% by weight of PTFE having a crystallization temperature not lower than 305° C. has been proposed (cf. e.g. Patent Document 6: Japanese Kokai Publication H07-70397). However, it is not certain whether this PFA composition, when used to cover electric wires by high-speed extrusion molding, can give covered electric wires showing no molding defect.